1. Field of the Invention
The present invention relates to a distributed constant type noise filter and, particularly, to a distributed constant type noise filter having superior high frequency characteristics in wide frequency band.
2. Description of the Prior Art
The digital technology is important technology that supports the IT (Information Technology) industry. The digital circuit technology for such as LSI, etc., has been utilized recently in not only computers and communication devices but also home electronic devices and car-mounted electronic devices.
However, high frequency current generated in an LSI is not confined in the vicinity of the LSI and is spread widely in a circuit substrate, such as to the printed circuit board on which the LSI is mounted so that high frequency current is inductively coupled to signal wiring and/or grounding wiring on the circuit substrate and leaked out as electromagnetic wave from a signal cable connected thereto, etc.
In a conventional circuit, such as an analog circuit, in which a portion is replaced by a digital circuit or a digital circuit having analog input/output, including an analog circuit and a digital circuit in a mixed state, the electromagnetic interference problem between the digital circuit and the analog circuit is serious.
As a countermeasure for such problem, it is effective to isolate the LSI, which is a source of high frequency current from a power supply system in high frequency range. This method is called xe2x80x9cpower de-couplingxe2x80x9d. As a power decoupling element, a noise filter such as a bypass capacitor has been used. Although the operational principle of the power de-coupling is simple, the development of a low impedance noise filter operable correspondingly to the high-speed operation of the digital circuit is substantially delayed. Particularly, the self-resonance phenomenon of the capacitor makes it difficult to keep impedance of the noise filter low in high frequency region.
Therefore, in order to accommodate to the digital circuit whose operation speed and hence operation frequency is increasing, a low impedance noise filter capable of maintaining a power de-coupling in high frequency range has been requested.
The noise filter of the conventional A.C. circuit takes in the form of a lumped constant type noise filter having 2-terminal construction that comprises a solid electrolytic capacitor, an electric double layer capacitor and a ceramic capacitor, mainly.
When the electric noise in the A.C. circuit in wide frequency range is to be removed by means of such capacitors, it has been usual that a plurality of capacitors of different kinds, such as aluminum electrolytic capacitor, tantalum capacitor and ceramic capacitor, whose self resonance frequencies are different, are provided in the A.C. circuit.
However, in the conventional noise filter, it has been troublesome to select a plurality of noise filter elements used for removal of electric noise in wide frequency range.
Furthermore, there has been another problem that the cost of the noise filter is increased due to the provision of the different kinds of noise filter elements.
An object of the present invention is to provide a noise filter capable of removing a wide frequency range and hence filtering out electric noise in high frequency band, which is generated by electron devices in high frequency range, by using a single noise filter element.
According to the present invention, a distributed constant type noise filter of a 3-terminal system having a transmission line structure is obtained.
That is, according to the present invention, a distributed constant type noise filter having a virtual distributed constant circuit capable of producing impedance, which is continuous in wide frequency range by controlling a width, length and thickness of a capacitor-forming portion thereof, is provided.
In more detail, the distributed constant type noise filter according to the present invention basically includes a capacitor portion constructed with a substantially flat metal plate and a pair of substantially flat dielectric members sandwiching the flat metal plate, a cathode terminal connected to the capacitor-forming portion, electrode portions formed by portions of the metal plate, which protrude from the capacitor-forming portion and anode terminals electrically connected to the electrode portions. It is a feature of the present invention that a length L and a width W of the capacitor-forming portion and an effective thickness h of the dielectric member of the capacitor-forming portion are determined on the basis of dielectric constant of the dielectric member.
As another embodiment of the present invention, the distributed constant type noise filter constructed as mentioned above is featured by that a ratio of the width W of the capacitor-forming portion to the thickness h of the dielectric member thereof and the length L of the capacitor-forming portion are determined on the basis of dielectric constant of the capacitor-forming portion.
With this construction, it is possible to reduce an impedance of the noise filter throughout a wide frequency range. Further, since the impedance on the high frequency side can be reduced, it is possible to provide a distributed constant type noise filter capable of removing electric noise in a wide frequency range, particularly, high frequency range.
As another embodiment of the present invention, the distributed constant type noise filter constructed as mentioned above is featured by that a surface area S2 of each anode terminal connected to the electrode portion is larger than a contact area Si of the anode terminal and the electrode portion.
With such construction, it is possible to reduce the impedance of the noise filter compared with the case where S1 is equal to S2.
In this construction, the distributed constant type noise filter is featured by that width W2 of the electrode portion connected to the power source is larger than width W1 of the electrode portion connected to the electron device.
In general, impedance of a D.C. power source is low and impedance of a power source terminal of a LSI is high. Therefore, with this construction, impedance on the load side in which a LSI is connected becomes high, so that the impedance matching is achieved. Therefore, it is possible to introduce electric noise to the distributed constant type noise filter of the present invention to thereby easily attenuate the noise.
In the distributed constant type noise filter according to the present invention, at least one recess may be formed in each longer side face of the capacitor-forming portion. In such case, a difference of impedance occurs within the capacitor-forming portion, with which the attenuation of electric noise can be improved.
In the distributed constant type noise filter according to the present invention, the capacitor-forming portion is featured by that one of the width W2 of the electrode portion connected to the power source and the width W1 of the electrode portion connected to the electron device is smaller than the other.
With such construction, it is possible to provide a high impedance region in the capacitor-forming portion to form a xcfx80 type filter circuit, so that the electric noise attenuation can be improved.
Furthermore, in the distributed constant type noise filter having the recess, the capacitor-forming portion is featured by that the capacitor-forming portion has a configuration having at least one bent portion to form a comb configuration. With such configuration of the capacitor-forming portion, the transmission line length, that is, the long side of the capacitor-forming portion can be increased, so that the electric noise attenuation can be improved.
In the described distributed constant type noise filter according to the present invention, the capacitor-forming portion may be a solid electrolytic capacitor, an electric double layer capacitor or a lamination of two electric double layer cells. With such construction of the capacitor-forming portion, it is possible to improve the breakdown voltage in the capacitor-forming portion.
In the distributed constant type noise filter according to the present invention, lead lines may be connected to the each electrode portion and the cathode terminal. In such case, its impedance is slightly higher, but it is more convenient for mounting.